 The primary productivity of an ecosystem can be reduced when there is an insufficient supply of a limiting nutrient  A limiting nutrient is an example of a limiting factor – a factor that causes population growth to decrease

 A density-dependent factor is a limiting factor that depends on a population size  Only become limiting when the population density reaches a certain level  Operate strongly when a population is large and dense

 Density-dependent limiting factors include competition, predation, parasitism, and disease

 Density-dependent limiting factor  Occurs when populations become crowded and organisms must compete for resources  More Individuals – More Competition  Less Individuals – Less Competition  Competition between members of species is a force behind evolutionary change

 When two species compete for the same resources, both species are under pressure to change in ways that decrease their competition  Over time, the species may evolve to occupy separate niches, since no two species can occupy the same niche in the same place at the same time

 Controls populations in nature  Occurs within a predator-prey relationship  This cycle can be repeated indefinitely

Predator-Prey Relationship

 Limit the growth of a population  Parasitic organisms range from microscopic bacteria to 30 cm long tapeworms  Similar to predators: take nourishment at weakening host

 Affect all populations, regardless of size  Examples: -unusual weather -natural disasters -seasonal cycles -human activities: damming rivers clear-cutting forests

 In response to density-independent limiting factors, species show a crash in population size, which may build up again or stay low for a long time  Storms can extinguish a population  Extreme weather can affect populations  Drought can affect populations of vegetation, which in turn affect the populations of consumers  Populations adapt to change  Major upsets can lead to long-term declines in populations, some of these upsets being caused by humans

 Competition  Predation  Parasitism  Disease

 Predator-Prey Relationship

 Unusual weather  Natural disasters  Seasonal cycles  Human activities: damming rivers clear-cutting forests

 Environments are always changing.